1. Field of the Invention
The present invention generally relates to a circuit for controlling an electric motor, and more specifically a circuit for controlling a DC motor which is suitable for use in driving window glasses in a power window system of a vehicle.
2. Prior Art
Referring now to FIG. 1, a conventional circuit for controlling a restraint torque of a DC motor which has been applied by the present inventor and accorded under JP-A-1-311883 as a laid-open number in Japan is generally shown. A DC power supply 1 is connected between a Vcc line and ground or GND. A DC motor 2 is connected between the Vcc line of the power supply and its drain of an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 3 for driving the DC motor 2 via a relay terminal 4. The MOSFET 3 also has its source connected to ground and its gate connected to a collector of a PNP transistor 5. The PNP transistor 5 has its emitter connected to the Vcc line and its base connected to an input terminal 6 via a resistor 7. A negative trigger pulse P1 is provided to the input terminal 6 and the PNP transistor 5 is turned on to turn on MOSFET 3. The MOSFET 3 in an ON state provides a current to the DC motor 2 and the motor 2 rotates during the negative trigger pulse.
A comparator 8 for maintaining the rotation of the DC motor 2 is connected between the Vcc line and ground a via supply bus not shown. The comparator has its inverting input connected to the relay terminal 4 or the drain of the MOSFET 3, its non-inverting input connected to a variable terminal of a variable resistor 9 connected between the Vcc line and ground, and an output connected to the gate of the MOSFET 3 via a resistor 10. The comparator 8 compares the terminal voltage V.sub.T of the relay terminal 4 with the reference voltage V.sub.R. The terminal voltage V.sub.T is defined to subtract the induced voltage V.sub.I from the Vcc. The comparator 8 outputs a logical "H" when V.sub.T &lt;V.sub.R. Therefore, once the negative trigger pulse P1 is applied to the trigger terminal 6, the MOSFET 3 is turned on, the drain voltage V.sub.T of the terminal 4 becomes a saturated voltage lower than that of the reference voltage V.sub.R and the comparator provides "H" to the gate of the MOSFET 3 to latch up the MOSFET 3 and to maintain the rotation of DC motor 2.
The DC motor 2 generates pulses each having an absolute induced voltage V.sub.I depending upon the rotation rate during the rotation thereof. Each pulse has a pair of negative and positive pulse components when a rotor of the DC motor 2 upon rotating comes to a predetermined angular position relative to a stator thereof. Even if the MOSFET 3 does not provide momentary power to the rotating DC motor 2, the rotating DC motor 2 will be rotated by a inertia and will generate the pulses.
An NPN transistor 11 is provided to periodically turn off the MOSFET 3 and to check the rotation condition of the DC motor 2 incorporated with the comparator 8. The NPN transistor 11 has its collector connected to the gate of MOSFET 3, its emitter connected to ground and its base connected to a clock terminal 12 via a resistor 13. Positive clock pulses P2 are periodically applied to the clock terminal 12 to provide a comparison period and the NPN transistor 11 is turned on to periodically turn off the MOSFET 3.
Upon applying the positive clock pulse, the comparator 8 can compare the terminal voltage V.sub.T, which will not interfere with the drain voltage of the MOSFET 3, with reference voltage V.sub.R. If V.sub.T is lower than V.sub.R, then the comparator 8 provides a "H" level to the gate of the MOSFET 3 after ending the compared period and the MOSFET 3 returns to the latch condition. If the V.sub.T is higher than V.sub.R, then the comparator 8 provides a "L" level to the gate of the MOSFET 3 after ending the compared period and the MOSFET 3 maintains a turn off condition. Supposing that the DC motor 2 controlled by the MOSFET 3 drives a window glass to close a door window of the vehicle, and the window glass is moving for example up to close the window by the trigger pulse, then DC motor 2 maintains the rotation by the inertia notwithstanding the applied periodical positive clock pulses unless the window glass meets with an obstacle against a frame of the door.
If the periodical clock pulses are not applied to the terminal 12, it is an unfavorable condition for the DC motor 2 that the window glass meets with an obstacle against a frame of the door because the rotating DC motor 2 receiving the power must be stopped to prevent damage by the obstacle. Also, internal coils of the DC motor 2 may be burned and cut by a static current through the DC motor 2.
However, by applying the periodical clock pulses to the terminal 12, an DC motor 2 does not generate the induced voltage when stopped. Accordingly the latch condition provided between the comparator 8 and the MOSFET 3 for supplying the power thereto is released.
Referring now to FIG. 2, a second conventional circuit for controlling a DC motor which also has been applied by the present inventor and accorded under JP-patent application No. 2-5401 filed on Jan. 23, 1990 is generally shown. In FIG. 2, components corresponding to those of FIG. 1 allocate the same numerals, respectively and detail descriptions are omitted. A resistor 14 is connected between the relay terminal 4 and the inverting input of the comparator 8. Two resistors 15 and 16 each connected to one of fixed terminals of the variable resistor 9 are connected to the Vcc line and ground, respectively to provide a finer adjustment of the reference voltage V.sub.R than that of FIG. 1. A wired AND function of the resistor 10 and the NPN transistor 11 in FIG. 1 is substituted with a two input AND gate 17. The AND gate 17 has its output connected to the gate of MOSFET 3, one input connected to the output of the comparator 8, and another input connected to an oscillator 18 for providing the negative periodical clock. Also, the PNP transistor 5 not shown in FIG. 2 may be added with its collector connected to the gate of MOSFET 3, its emitter connected to the Vcc line and its base connected to the input terminal 6 via the resistor 7 as shown in FIG. 1.
An operation of the second conventional control circuit for a DC motor as shown in FIG. 2 is identical to that of FIG. 1, so a detailed description of the operation is omitted.
These control circuits are advantageous compared to a conventional circuit having a series resistor passing through a supply current to a DC motor because the series resistor can be employed for monitoring a DC motor current but has a serious loss of the current which degrades the efficiency of the power energy supplied thereto.
Such a motor 2 is generally employed as a drive means for a power or automatic window, while a secondary battery is used as a DC power supply of the vehicle. The battery has many electrical connections to equipments such as a cooler, a dynamo, head lights, a car-radio machine, etc. installed in the vehicle. The voltage of the Vcc line connected to the battery is undesirably changed when any equipment is turned on or off. This change of Vcc causes a change of a restraint torque of a DC motor, that is, higher power supply voltage causes an increasing restraint torque, while lower power supply voltage causes a decreasing restraint torque of the motor.
In FIGS. 1 and 2, a critical or minimum current I.sub.min whether rotation of the DC motor 2 is maintained or not is as follows. EQU I.sub.min =(Vcc-V.sub.I) /Rm=V.sub.R /Rm
wherein, Vcc denotes a power supply voltage, V.sub.I denotes a induced voltage, and Rm denotes an internal resistance of the DC motor 2. Therefore, the restraint torque depends upon the critical current I.sub.min. The critical current changes due to a change of the reference voltage V.sub.R and in turn a change of the power supply voltage Vcc.
The conventional circuits as shown in FIGS. 1 and 2, can control the restraint torque of the motor but not the rotation rate or speed. It is not practical to control the rotation rate by using a dynamo directly connected to a shaft of the DC motor because of high cost.
The conventional DC motor 2 as shown in FIG. 1 or 2 generally needs a direct current (DC) power supply. In the vehicle such as an automobile, it is frequent that the DC power supply is obtained from an AC power supply such as an alternator or dynamo with a rectifying circuit for providing a pulsating power supply from the AC power supply and a smoothing capacitor or battery for smoothing the pulsating power as indispensable means. The oscillator 18 comprising an unstable mutivibrator by using a 555 available from Signetics or a crystal oscillator and clock dividers, should be omitted because the circuit is more complex and the mean time between failures (MTBF) will be increased if the oscillator 18 is omitted. The power consumed by the control circuit, mainly the pulse oscillator and comparator is not be negligible.